The present invention relates to color organic light emitting diode (OLED) video displays. In particular, the present invention is directed to color conversion elements incorporating semiconductor nanocrystals which are dispersed in a transparent binding material. Fabricated independently from the OLED layers, the color conversion elements absorb light that is emitted from the OLED layers at a first wavelength and re-emit this light at a different wavelength. The color conversion elements of the present invention may be employed in either down-emitting or up-emitting color OLED display devices.
Organic light emitting diodes (xe2x80x9cOLEDsxe2x80x9d) have been known for approximately two decades. All OLEDs work on the same general principles. One or more layers of semiconducting organic material are sandwiched between two electrodes. An electric current is applied to the device, causing negatively charged electrons to move into the organic material(s) from the cathode. Positive charges, typically referred to as holes, move in from the anode. The positive and negative charges meet in the center layers (i.e., the organic material), recombine, and produce photons. The wavelength of the photonsxe2x80x94and consequently the color of the emitted lightxe2x80x94depends on the electronic properties of the organic material in which the photons are generated.
In a typical OLED, at least one of the electrodes is transparent. The cathode may be constructed of a low work function material. The holes may be injected from a high workfunction anode material into the organic material. Typically, the devices operate with a DC bias of from 2 to 30 volts. The films may be formed by evaporation, spin casting, self-assembly or other appropriate film-forming techniques. Thicknesses typically range from a few mono layers to about 2,000 Angstroms.
In a typical passive matrix-addressed OLED display numerous OLEDs are formed on a single substrate and arranged in groups in a regular grid pattern. Several OLED groups forming a column of the grid may share a common cathode, or cathode line. Several OLED groups forming a row of the grid may share a common anode, or anode line. The individual OLEDs in a given group emit light when their cathode line and anode line are activated at the same time.
An OLED may be designed to be viewed either from the xe2x80x9ctopxe2x80x9d xe2x80x94the face opposite the foundational substratexe2x80x94or from the xe2x80x9cbottomxe2x80x9d i.e., through the substrate, from the face opposite the light emitting layer. Whether the OLED is designed to emit light through the top or the bottom, the respective structure between the viewer and the light emitting material needs to be sufficiently transparent, or at least semi-transparent, to the emitted light. In many applications it is advantageous to employ an OLED display having topside light output. This permits the display to be built on top of a silicon driver chip for active matrix addressing.
The color of light emitted from the OLED display device can be controlled by the selection of the organic material. Specifically, the precise color of light emitted by a particular structure can be controlled both by selection of the organic material as well as by selection of luminescent impurities or dopants, added to the organic materials. By changing the kinds of organic solids making up the light-emitting layer, many different colors of light may be emitted, ranging from deep blue to red.
The color of light emitted from an OLED display device may be affected not only by the source material and/or doping of the light emitting layer, but also by color filters and color converters or color changing films that are formed above the OLED pixels or light emitting layers.
OLEDs have a number of beneficial characteristics. These include a low activation voltage (about 5 volts), fast response when formed with a thin light-emitting layer, and high brightness in proportion to the injected electric current. OLEDs are currently the subject of aggressive investigative efforts.
Although substantial progress has been made in the development of OLEDs to date, additional challenges remain. For example, there are drawbacks to the various existing approaches to the fabrication of the components that generate colored light in OLED displays. One approach provides a self-emissive pixelated display with RGB subpixels placed next to each other. This approach, in principle, allows the best possible performance because no light is lost through filter absorption or color conversion. It requires, however, precise shadow mask fabrication and alignment in the process of vacuum deposition for displays using low-molecular-weight material. Such precision in the fabrication of shadow masks is technologically difficult for miniature, high-resolution displays with pixel sizes in the range of several microns.
In a second design approach for making color OLEDs, pixels emitting white light are combined with precisely aligned color filter elements. The white light is changed to the color of the particular color filter. The color filters can be inefficient, however, because the filters inevitably absorb some light.
A third approach aligns photoluminescing color conversion elements with pixels emitting near ultraviolet or blue light.
The latter two approaches are technologically feasible given the present state of the art because all pixels emit the same color and the filter media can be patterned and aligned to the OLED pixels. When the relevant layers have high quantum efficiency of photoluminescence and internal losses are minimized, the third, or color conversion, technique provides better efficiency.
But even the color conversion technique has drawbacks. Most materials used for color conversion have broad emission photoluminescence spectra that require additional optical filters for spectra correction. These additional optical filters on top of the color conversion materials introduce additional loss of intensity.
In the present invention, Applicant presents an effective new design for color conversion through fabrication of appropriate, inorganic-based elements that provide narrow photoluminescence emission bands upon optical stimulation by a higher photon energy source. Semiconductor nanocrystals are known to have narrow and tunable emission bands which are determined very specifically by their size, yet are not dependent on the details of the near ultraviolet or blue excitation spectrum. This permits all three color conversion elements (red, green and blue) to be pumped by a single excitation source, for example, the organic electroluminescence display matrix element. Semiconductor nanocrystals (e.g., passivated CdSe) are widely tunable in the size range of approximately 10 to 200 Angstroms, which covers optical conversion through the visible spectrum, and can be controllably fabricated with narrow size distributions from scalable colloidal precipitation and other techniques known in the prior art.
In the present invention, the layer or layers containing the semiconductor nanocrystals are fabricated independently from the OLED layers. Stable films of semiconductor nanocrystals can be patterned using standard photolithographic techniques unlike the OLED layers which are sensitive to humidity and other environmental variations.
Previous approaches for color conversion have focused specifically on the use of organic dye molecular systems such that the red, green and blue color converters may require completely different synthetic routes, thus increasing manufacturing complexity. For example, U.S. Pat. No. 5,126,214 to Tokailin et al. discloses the use of an organic electroluminescent matrix element, together with a fluorescent material part that corresponds functionally to the color conversion element of the present invention. The fluorescent material part emits a fluorescence in a visible light range from red to blue. Tokailin et al. disclose the use of fluorescent dye materials to provide color conversion. In contrast, the present invention uses semiconductor nanocrystals for color conversion.
Similarly, U.S. Pat. No. 5,294,870 to Tang et al. contains reference to both organic and inorganic dye materials that perform color conversion.
The benefits of the present invention relate directly to fundamental properties of semiconducting nanocrystals as described in the available research literature.
For example, a review of the status of semiconductor quantum nanocrystal research is presented by A. P. Alivisatos in xe2x80x9cSemiconductor clusters, nanocrystals, and quantum dots,xe2x80x9d Science 271 (Feb. 16, 1996) 933-937. In addition, an article by C. B. Murray, D. J. Norris and M. G. Bawendi, xe2x80x9cSynthesis and characterization of nearly monodisperse CdE (E=S, Se, Te) semiconductor nanocrystallites,xe2x80x9d J. Am. Chem. Soc. 115 (1993) 8706-8715, contains detailed art about the synthesis and properties of nanocrystals from the cadmium family.
Methods of fabricating semiconductor nanocrystals are disclosed in U.S. Pat. No. 5,559,057 to Goldstein (method of manufacturing thin films from nanocrystal precursors) and U.S. Pat. No. 5,525,377 to Gallagher et al. (method of making doped encapsulated semiconductor nanocrystallites for use in thin films for electroluminescent displays.)
The beneficial properties of semiconductor nanocrystals in application to color OLED display technology include the following: the photoluminescence color of semiconducting nanocrystals depends proportionally on their size. This assures tunability of photoluminescence and permits direct determination of nanosphere distribution from the position and width of the photoluminescence spectrum. This provides an analytical process benefit and is a key enabler. In the present invention, precise control of the nanocrystal size distribution defines the capability for color conversion to the red, green and blue that form subelements comprising, each color pixel of the integrated display.
In addition, delocalization of the electronic states over each nanosphere, coupled with passivation techniques, support immunity from local film environmental details. This provides better stability, especially in comparison with known organic strategies to achieve color conversion.
Nanocrystals of different sizes manifest a broad excitation spectrum which permits conversion to red, green and blue colors without the requirement for color-specific pumping; namely, a single electroluminescent matrix element can generate all the light required to permit conversion by the nanocrystal films into red, green and blue subpixel elements. This permits integration simplicity in the OLED device design.
Another beneficial attribute relates to the synthesis of semiconductor nanocrystals via organometallic precursor injection. This type of synthesis is independent of the reaction vessel, therefore it is scalable, which can reduce manufacturing cost. Alternative synthesis processes such as chemical vapor deposition are also available, giving more options for manufacturing flexibility.
Patterning of nanocrystal films may be done using standard photolithographic art. This is another cost-effective enabler.
The known art comprises OLED display devices that utilize semiconductor nanocrystals to emit light of different wavelengths. For example, U.S. Pat. No. 5,537,000 to Alivisatos et al. discloses an electroluminescent device with a semiconductor nanocrystal electron transport layer capable of emitting light of various wavelengths. This design contemplates production of different colors of emitted light by varying the voltage applied to the device, or, as in the present invention, by varying the size or type of semiconductor nanocrystal. The design disclosed by Alivisatos et al. combines the semiconductor nanocrystals with the organic electron transport layer, which together emit the light of different wavelengths. In contrast, the present invention provides the semiconductor nanocrystals in a separate layer or layers that are fabricated independently from the OLED layers. In Alivisatos et al., the nanocrystal/OLED layer emits the light at different wavelengths. In the present invention the OLED layer emits light at one wavelength (preferably blue). Then the semiconductor nanocrystal layer or layers absorb the light that is emitted by the OLED layers and re-emit that light a different wavelengths, to produce red and green colors. The blue light emitted by the OLED layers of the present invention may be re-emitted by the semiconductor nanocrystal layer or layers at a different wavelength. In the present invention, the separation of the fabrication of the light emitting OLED layers from fabrication of the color conversion layers makes it possible to manufacture miniature displays cost-effectively.
U.S. Pat. No. 5,677,545 to Shi et al. discloses an OLED with a coupling layer between two organic layers. The coupling layer may comprise fluorescent nanocrystals whose size may tune the color of the light emission. Like the Alivisatos et al. design, the Shi et al. design emits light of various colors from the nanocrystal/OLED layers. This presents manufacturing obstacles that are overcome in the present invention, which separates the fabrication of the OLED layers from that of the semiconductor nanocrystal layer or layers.
It has been a continuing challenge to devise OLED structures which provide superior color displays, at maximum efficiency, all in a miniature environment. Accordingly, there is a need for a miniature OLED design that provides a more effective means of color conversion of emitted light. There is also a need for a more efficient method to effect the conversion of high photon energy electroluminescence from an OLED display element into lower photon energy photoluminescence in order to build an integrated color organic electroluminescence display device. There is also a need for an improved method of fabricating an integrated color display device onto standard substrates such as glass or silicon, using standard photolithographic techniques. The present invention meets these needs, and provides other benefits as well.
It is therefore an object of the present invention to provide an economical manufacturing process for a color OLED display device.
It is another object of the present invention to provide a color OLED display device made by a scalable synthetic process that allows manufacturing cost reductions.
It is still another object of the present invention to provide a color OLED display device which may be made by an alternative synthetic process such as chemical vapor deposition.
It is yet another object of the present invention to provide an improved method for fabricating a color OLED display device onto standard substrates.
It is a further object of the present invention to provide an integrated color OLED display device using standard photolithographic techniques.
It is still a further object of the present invention to provide an integrated color OLED display device in which high photon energy electroluminescence from the OLED display element is converted into lower photon energy photoluminescence.
It is yet a further object of the present invention to provide a color OLED display device with efficient color conversion means.
It is another object of the present invention to provide a color OLED display-device with color conversion elements that can be fabricated using standard photolithographic techniques.
It is still another object of the present invention to provide a color OLED display device with color conversion elements that have narrow photoluminescence bands.
It is yet another object of the present invention to provide a color OLED display device with color conversion elements that have widely tunable emission bands.
It is a further object of the present invention to provide a color OLED display device with inorganic-based color conversion elements.
It is still a further object of the present invention to provide a color OLED display device using films of inorganic semiconductor nanocrystals as the color conversion elements.
It is yet a further object of the present invention to provide a color OLED display device with color conversion elements that are not dependent on the details of the near ultraviolet or blue excitation spectrum.
It is another object of the present invention to provide a color OLED display device wherein all three color conversion elements can be pumped by a single excitation source.
It is yet another object of the present invention to provide a color OLED display device with color conversion elements that can be controllably fabricated with narrow size distributions.
It is still another object of the present invention to provide a color OLED display device with color conversion elements that can be fabricated from scalable colloidal precipitation.
It is a further object of the present invention to provide a color OLED display device with an efficient xe2x80x9cdown-emittingxe2x80x9d display.
It is yet a further object of the present invention to provide a color OLED display device with an efficient xe2x80x9cup-emittingxe2x80x9d display.
It is still a further object of the present invention to provide a color OLED display device with color conversion elements that permit direct determination of the nanosphere distribution from the position and width of the photoluminescence spectrum.
It is another object of the present invention to provide a color OLED display device with improved stability of the color conversion elements.
It is still another object of the present invention to provide a color OLED display device having color conversion elements that do not require color-specific pumping.
It is yet another object of the present invention to provide a color OLED display device with greater integration simplicity in its color conversion.
It is a further object of the present invention to provide a method of making a color OLED display device that permits fabrication of the light-emitting OLED layers independent of the fabrication of the light-absorbing and re-emitting semiconductor nanocrystal layer or layers.
Additional objects and advantages of the invention are set forth, in part in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.
As illustrated in the accompanying drawings and disclosed in the accompanying claims, the invention is an integrated organic light emitting diode (OLED) color display device. The OLED color display device comprises at least one pixel element capable of emitting visible light of varying wavelengths.
A pixel element for the organic light emitting diode color display device may comprise: a substrate; a transparent element through which the visible light is emitted; an addressable two-dimensional organic light emitting diode matrix that emits monochrome light, in contact with at least one of the substrate and the transparent element; a color conversion matrix element for conversion of the wavelength of the monochrome light, in contact with the organic light emitting diode matrix and at least one of the substrate and the transparent element; and a cover element for protecting the color conversion matrix element and the organic light emitting diode matrix against physical, chemical or thermal damage, in contact with at least one of the color conversion matrix element and the organic light emitting diode matrix. The color conversion matrix element may absorb the monochrome light emitted from the organic light emitting diode matrix and re-emit the visible light at different wavelengths.
The pixel element may further comprise a two-dimensional array of color conversion elements. The color conversion matrix element may further comprise at least one red, at least one green and at least one blue color converting subelement and a plurality of semiconductor nanocrystals uniformly dispersed in a transparent organic binding material. The color conversion matrix element may be fabricated independently from the organic light emitting diode matrix. The color conversion matrix element may comprise at least one layer. The semiconductor nanocrystals may be selected from the group of the semiconductor compounds CdS, CdSe and CdTe and mixtures of two or more of the semiconductor compounds.
In the device of the present invention, the color of the visible light re-emitted by the color conversion matrix element is tunable by altering the size of the semiconductor nanocrystals and the size distribution of the semiconductor nanocrystals is precisely controlled to define the capability for color conversion to a particular wavelength of re-emitted light.
The red color converting subelement may further comprise semiconductor nanocrystals that absorb the monochrome light emitted from the organic light emitting diode matrix and re-emit the monochrome light as red light. The green color converting subelement may further comprise semiconductor nanocrystals that absorb the monochrome light emitted from the organic light emitting diode matrix and re-emit the monochrome light as green light. The blue color converting subelement may further comprise at least one of (a) the transparent organic binding material without semiconductor nanocrystals such that the blue color converting subelement is transparent to the monochrome light; and (b) an optical filter without semiconductor nanocrystals, capable of spectrum correction and transparent to the monochrome light. In an alternate embodiment the blue color converting subelement may comprise semiconductor nanocrystals that absorb the monochrome light emitted from the organic light emitting diode matrix and re-emit the monochrome light as a different wavelength of blue light.
In a first embodiment of the present invention, the color conversion matrix element is formed in contact with the substrate, which is the transparent element and the cover is formed in contact with the organic light emitting diode matrix. In this embodiment, the pixel element is a component of a xe2x80x9cdown-emittingxe2x80x9d OLED display device.
In a second embodiment of the present invention, the organic light emitting diode matrix is formed in contact with the substrate; the substrate is opaque; the cover is formed in contact with the color conversion matrix element; and the cover is the transparent element. In this alternate embodiment, the pixel element is a component of an xe2x80x9cup-emittingxe2x80x9d OLED display device.
The present invention is also directed to a color display device for providing an image utilizing light emitted from at least one pixel element capable of emitting visible light of varying wavelengths. The display device may comprise: a substrate; a transparent element through which the visible light is emitted; an addressable two-dimensional organic light emitting diode matrix that emits monochrome light, in contact with at least one of the substrate and the transparent element; a color conversion matrix element for conversion of the wavelength of the monochrome light, in contact with the organic light emitting diode matrix and at least one of the substrate and the transparent element; and wherein the color conversion matrix element (a) may absorb the monochrome light emitted from the organic light emitting diode matrix and re-emit the visible light at different wavelengths; (b) may further comprise a two-dimensional array of color conversion elements; (c) may further comprise at least one red, at least one green and at least one blue color converting subelement; (d) may further comprise a plurality of semiconductor nanocrystals uniformly dispersed in a transparent organic binding material; (e) may further comprise at least one layer; and (f) may be fabricated independently from the organic light emitting diode matrix; and a cover element for protecting the color conversion matrix element and the organic light emitting diode matrix against physical chemical or thermal damage, in contact with at least one of the color conversion matrix element and the organic light emitting diode matrix.
In the display device, the semiconductor nanocrystals are selected from the group of the semiconductor compounds CdS. CdSe and CdTe and mixtures of two or more of the semiconductor compounds. The color of the visible light re-emitted by the color conversion matrix element is tunable by altering the size of the semiconductor nanocrystals and the size distribution of the semiconductor nanocrystals is precisely controlled to define the capability for color conversion to a particular wavelength of re-emitted light.
In the display device, the red color converting subelement may further comprise semiconductor nanocrystals that absorb the monochrome light emitted from the organic light emitting diode matrix and re-emit the monochrome light as red light; the green color converting subelement may further comprise semiconductor nanocrystals that absorb the monochrome light emitted from the organic light emitting diode matrix and re-emit the monochrome light as green light; and the blue color converting subelement may further comprise at least one of (a) the transparent organic binding material without semiconductor nanocrystals such that the blue color converting subelement is transparent to the monochrome light; and (b) an optical filter without semiconductor nanocrystals, capable of spectrum correction and transparent to the monochrome light.
In an alternate embodiment of the display device of the present invention, the blue color converting subelement may comprise semiconductor nanocrystals that absorb the monochrome light emitted from the organic light emitting diode matrix and re-emit the monochrome light as a different wavelength of blue light.
In a first embodiment of the display device of the present invention, the color conversion matrix element is formed in contact with the substrate; the substrate is the transparent element; and the cover is formed in contact with the organic light emitting diode matrix. In this embodiment, the display device is a xe2x80x9cdown-emittingxe2x80x9d OLED display device.
In a second embodiment of the display device of the present invention, the organic light emitting diode matrix is formed in contact with the substrate; the substrate is opaque; the cover is formed in contact with the color conversion matrix element; and the cover is the transparent element. In this embodiment, the display device is an xe2x80x9cup-emittingxe2x80x9d OLED display device.
The present invention is also directed to a method of fabricating an integrated organic light emitting diode color display device comprising at least one pixel element capable of emitting visible light of varying wavelengths, comprising the steps of: forming a substrate; forming a transparent element through which the visible light is emitted; forming an addressable two-dimensional organic light emitting diode matrix that emits monochrome light in contact with at least one of the substrate and the transparent element; forming a color conversion matrix element for conversion of the wavelength of the monochrome light, in contact with the organic light emitting diode matrix and at least one of the substrate and the transparent element; wherein the color conversion matrix element may absorb the monochrome light emitted from the organic light emitting diode matrix and re-emit the visible light at different wavelengths; and forming a cover element for protecting the color conversion matrix element and the organic light emitting diode matrix against physical, chemical or thermal damage, in contact with at least one of the color conversion matrix element and the organic light emitting diode matrix. The step of forming the color conversion matrix element is independent from the step of forming the organic light emitting diode matrix. The method may further comprise the step of providing the color conversion matrix element with a plurality of semiconductor nanocrystals uniformly dispersed in a transparent organic binding material. The method may further comprise the step of altering the size of the semiconductor nanocrystals in order to tune the color of the visible light re-emitted by the color conversion matrix element. The method may further comprise the step of precisely controlling the size distribution of the semiconductor nanocrystals in order to define the capability for color conversion to a particular wavelength of re-emitted light.